High-density recording is feasible by using a laser for a read/write operation since such an operation is achieved by, in principle, focusing light on a spot having a diameter substantially equal to the wavelength of the laser used. Typically used in the optical recording are an optical disk (hereinafter referred to as "OD") as an information recording medium and an optical pickup using a laser system as means for achieving writing to and reading out of the OD.
Pits provided in the OD for the recording are about 0.9 .mu.m in size each, and lines of pits, namely tracks, are arranged with a narrow pitch of about 1.6 .mu.m. Therefore, in a practical pickup operation pits must be scanned while optical pickup is adjusted to keep on a right track without straying perpendicular to the track direction and on a right focus against irregular revolution of the OD.
A conventional pickup employs an optical element, such as a half mirror or a hologram, as a beam splitter and is adapted to split light reflected from an OD to pick up pits. In this case, a gap from a right track when it strays perpendicular to the track direction in the plane of the OD is corrected by, for example, the 3-beam method in which light from one laser diode (hereinafter referred to as "LD") as a light source is split into three beams by a diffraction grating while a focusing error is detected by, for example, the astigmatism method using a cylindrical lens.
Alternatively, there is developed a SCOOP (Self Coupled Optical Pickup) method as disclosed in, for example, Japanese Unexamined Patent Publication No. 72688/1991, wherein light reflected by an OD is returned to an LD (such light will be referred to as "returning light") and a signal is picked up by utilizing a change in oscillation state of the LD due to returning light. The objective of the SCOOP method is to reduce the number of optical components, such as a beam splitter and a cylindrical lens, as used in the aforesaid conventional pickup, to reduce the cost, and to facilitate precise positioning.
In this SCOOP method are used, for example, five LDs 41 to 45 which are linearly arranged as shown in FIG. 8. The central LD 43 is for RF (Radio Frequency) signals LDs 42 and 44 on both sides of LD 43 are each for tracking error signals, and the outermost LDs 41 and 45 are each for focusing error signals and are slightly shifted in opposite directions along the optical axis. These LDs for error signals detect tracking errors and focusing errors. LDs 42 and 44 for tracking errors are required to detect each of the sides of a signal track and, hence, a semiconductor laser system is configured so that a line of laser beam spots 46 to 50 obliquely crosses the track to detect a tracking error.
The aforesaid semiconductor laser system for use in an optical pickup of the SCOOP type needs to have, besides an LD for recorded signals, LDs for detecting tracking and focusing errors in a fixed positional relation therebetween. Practically, it is very difficult to three-dimensionally dispose separate chips of the order of micron. Therefore, a monolithic and compact semiconductor laser system has so far been desired.
There is another problem to be solved. Since LD 43 for recorded signal and LDs 42 and 44 for tracking error signal are disposed between LDs 41 and 45 for focusing error signal, the LDs 41 and 45 detect signals from portions significantly spaced apart from each other and a focusing error is corrected corresponding to such signals. This results in a problem of inaccurate correction of a focusing error.
The foregoing publication does not disclose the light-receiving part for detecting a variation in laser oscillation state of the reflected light. In case that a plurality of LDs emit light in a narrow region, however, it is required to construct the laser system so that light of each LD can be received by its associated light-receiving element with no mixture of light of another LD. For this reason, an optical waveguide groove must be formed between each LD and its associated light-receiving element, as disclosed in, for example, Japanese Unexamined Utility Model Publication No. 89273/1988. This results in an increased number of components used and hence in a laser system of increased size. Therefore, the problem is caused that the conventional laser systems for use in an optical pickup cannot achieve accurate error detection or reduction in their size.